1. Technical Field
The present disclosure relates to master oscillator systems and laser apparatuses.
2. Related Art
The miniaturization and increased levels of integration of semiconductor integrated circuits has led to a demand for increases in the resolutions of semiconductor exposure devices (called “exposure devices” hereinafter). Accordingly, advances are being made in the reduction of the wavelengths of light emitted from exposure light sources. Gas laser apparatuses are being used as exposure light sources instead of conventional mercury lamps. At present, a KrF excimer laser apparatus that emits ultraviolet light at a wavelength of 248 nm and an ArF excimer laser apparatus that emits ultraviolet light at a wavelength of 193 nm are being used as gas laser apparatuses for exposure.
Immersion exposure, in which the apparent wavelength of an exposure light source is reduced by filling the space between the exposure lens of an exposure device and a wafer with a liquid and changing the refractive index, is being researched as a next-generation exposure technique. In the case where immersion exposure is carried out using an ArF excimer laser apparatus as the exposure light source, the wafer is irradiated with ultraviolet light at a wavelength of 134 nm within the liquid. This technique is referred to as ArF immersion exposure (or ArF immersion lithography).
The natural oscillation amplitude of a KrF excimer laser apparatus, an ArF excimer laser apparatus, or the like is as wide as 350-400 pm. Accordingly, there are cases where chromatic aberration will occur if a projection lens is used in the exposure device, leading to a drop in the resolution. Accordingly, it is necessary to narrow the spectral bandwidth (spectral width) of the laser beam emitted from the gas laser apparatus until the chromatic aberration reaches a level that can be ignored. In recent years, the spectral width has been narrowed by providing a line narrow module having a line narrowing element (an etalon, a grating, or the like) within the laser resonator of the gas laser apparatus. A laser apparatus that narrows the spectral width in this manner is called a narrow-band laser apparatus.